Patient interfaces, such as masks in pressure support systems, are used for delivering gas to a user. Such gases like air, cleaned air, oxygen, or any modification thereof are submitted to the user (also referred to as patient) via the patient interface in a pressurized or unpressurized way.
For several chronic disorders and diseases the usage of such a patient interface is necessary or at least advisable.
One example of such a disease is obstructive sleep apnea or obstructive sleep apnea syndrome (OSA). OSA is usually caused by an obstruction of the upper airway. It is characterized by repetitive pauses in breathing during sleep and is usually associated with a reduction in blood oxygen saturation. These pauses in breathing, called apneas, typically last 20 to 40 seconds. The obstruction of the upper airway is usually caused by reduced muscle tonus of the body that occurs during sleep. The human airway is composed of walls of soft tissue which can collapse and thereby obstruct breathing during sleep. Tongue tissue moves towards the back of the throat during sleep and thereby blocks the air passages. OSA is therefore commonly accompanied with snoring.
Different invasive and non-invasive treatments for OSA are known. One of the most powerful non-invasive treatments is the usage of Continuous Positive Airway Pressure (CPAP) or Bi-Positive Airway Pressure (BiPAP) in which a patient interface, e.g. a face mask, is attached to a tube and a machine that blows pressurized gas, preferably air, into the patient interface and through the airway of the patient in order to keep it open. Positive air pressure is thus provided to a patient through a hose connected to a patient interface or respiratory interface, such as a face mask, that is worn by the patient regularly at night. The afore-mentioned long-term use of the patient interface is the result, since the wearing of the patient interface usually takes place during the sleeping time of the patient.
Examples for patient interfaces are:                nasal masks, which fit over the nose and deliver gas through the nasal passages,        oral masks, which fit over the mouth and deliver gas through the mouth,        full face masks, which fit over both, the nose and the mouth, and deliver gas to both, and        nasal pillows, which are regarded as masks as well within the scope of the present invention and which consist of small nasal inserts that deliver the gas directly to the nasal passages.        
In order to guarantee a reliable operation of the device, the patient interface needs to closely fit on the patient's face to provide an air-tight seal at the mask-to-face interface. The patient interface is worn using a headgear with straps that go around the back of the patient's head. These straps are often made of an elastic textile material. The patient interface or mask in practice usually comprises a soft cushion that is used as mask-to-patient interface, i.e. that contacts the face of the patient when the mask is worn, as well as it usually comprises a so-called mask shell building a rigid or semi-rigid holding structure for holding the cushion in place and for supplying mechanical stability to the patient interface.
The cushion usually comprises one or more pads made of gel or silicone or any other soft material in order to increase the patient comfort and guarantee a soft feeling on the patient's face. The latter-mentioned mask shell is usually made of polycarbonate and normally further comprises a hose interface that is adapted for connecting the air supplying hose to the mask. Depending on the type of the mask, it may also comprise a mechanism with an additional cushion support on the forehead (also denoted as forehead support) to balance the forces put by the mask around the airway entry features of the human face.
During use, mask materials can degrade and finally wear out. Such a degradation or wear-out may decrease the overall mask performance. During use, the headgear can lose at least a part of its elasticity and can change its mechanical properties, such as tensile strength and/or level of elongation under stress, all of which can result in reduced therapy compliance and patient comfort. There is especially a risk for higher leakage and skin damage or red marks, since higher strapping forces are needed to make a mask leak-tight on the face with a worn-out headgear. Although there is a possibility for OSA patients to regularly replace their mask or components thereof through new products, many patients do not make use of this option.
U.S. 2012/0285464 A1 discloses a headgear with straps that are configured to change colour in response to a level of force induced stress on the straps. This shall help to visually indicate the patient an over tightening of straps which could otherwise cause necrosis if not loosened. However, a tension indicator as disclosed in U.S. 2012/0285464 A1 is not essential for the patient, since the patients directly feel the tension and therefore usually recognize themselves when the headgear straps are over tightened. Visual inspection of the tension indicator may also be difficult while the headgear is being worn. On the other hand, a wear-out of parts of the patient interface is much more difficult to detect for a patient. Usually this may not be recognized in an intuitive way.
U.S. 2012/0285461 A1 discloses a mask for patient ventilation, wherein the mask includes a headgear with a side strap that may change colors or change from opaque to somewhat translucent in response to a level of force induced stress on the strap which is indicative of a level of force or strap tightening that is considered to be so tight as to cause necrosis if not loosened.
WO 2014/142681 A1 discloses a nasal cannula assembly with a headgear strap that includes a tightness indicator which provides the user with feedback regarding whether the tightness of the headgear strap is inside or outside a desired tightness range (too tight or too loose).
To overcome these problems, objective replacement indicators are required that are directly coupled to the level of wear-out. There is thus a need for objective and reliable wear-out indicators that may be easily applied in a patient interface and that give the patient a clear and easily comprehensible feedback when he/she should replace parts of the patient interface.